docs: def _edges_to_adjacency_list(edges):
      """Convert a list of edges into an adjacency list."""
      adj = {}
      for i, j in edges:
          if i in adj:  # pragma: no cover
              ni = adj[i]
          else:
              ni = adj[i] = set()
          if j in adj:
              nj = adj[j]
          else:
              nj = adj[j] = set()
          ni.add(j)
          nj.add(i)
      return adj